Photomask images or patterns, which define various elements in a photolithography process, are focused on a photoresist using light. To realize tiny features, finer images must be focused on the photoresist, and optical resolution must be increased. However, there are limits to the resolution that can be attained.
Reduction in a line width of a device results in a reduction of the size of a contact region. In general, a contact region is more difficult to form than line and space patterns. This problem has resulted in the development of various techniques of forming a fine contact hole. The size of a contact hole in dynamic random access memory (DRAM) is almost the same as the line width of an interconnect wiring, such as a bit line or a metal interconnection. However, formation of a contact hole pattern using lithography is still far more difficult than that of an interconnect wiring. That is, a fine contact hole of a desired size is difficult to form only using lithography. In general, a fine contact hole of a desired size is made by patterning a photoresist through a photo process and then performing an additional process, for example, a thermal flow process, on the patterned photoresist. Otherwise, a fine contact hole may be formed using various techniques in a lithography process, for example, a contact hole may be etched using a slope etch.
However, an increase in the number of processes for manufacturing a semiconductor device results in an increase in manufacturing costs. Further, as additional processes are involved, the degree of distribution of the line width of a device worsens. Accordingly, it is necessary to develop a resolution enhancement technique (RET) of directly transferring a fine contact hole of good quality onto a photoresist using a photolithography process.
Meanwhile, an alternating phase-shifting mask (PSM) allows the direct transfer of a fine contact hole pattern onto a photoresist while securing reproducibility and stability. In general, it is possible to form a fine contact hole with better resolution and obtain a more sufficient process margin by using the alternating PSM, as opposed to using a binary mask or an attenuated PSM. Nevertheless, the alternating PSM has not been widely used mainly due to its three-dimensional (3D) structure. An alternating phase of a contact hole array is made by etching quartz. The use of the etched quartz, however, causes problems such as a 3D effect, a phase error, and phase conflict. Accordingly, technology and a method for preventing the occurrence of such problems are required to manufacture a photomask.